In European patent application, Publication No. 0113565, of which I am the inventor and the specification and drawings of which are hereby incorporated by reference, a filter unit is disclosed that comprises a rectangular sheet of filtering material and a pair of first and second superimposed frames. The first frame carries the filtering material and the second frame, which is designed to stretch and support the filtering material, is received within and below the first frame. The sheet of filtering material has a width and length greater than the width and length of the first and second frames and has a marginal portion surrounding a central portion. The marginal portion is folded over the first frame and doubled back upon the central portion. When the second frame is received within the first frame, the sheet of filtering material is held between the first and second frames at the marginal portion of the sheet of filtering material, with the second frame also stretching the sheet of filtering material so that it remains taut when supported by the frames.
The sheet of filtering material preferably has a rectangular form incorporating a set of four cut-off corners or vertices. Each of the cut-off corners are in the form of an isosceles triangle, and preferably are all of the same configuration. When the sheet of filtering material is folded over the first frame, the absence of the corners or vertices inhibits the bunching and bulging of the sheet of filtering material around the corners of the first frame. Such bulging or bunching of the sheet of filtering material is undesirable because it makes it difficult, if not impossible, to install an assembled filter unit formed of the two frames and the sheet of filtering material into a U-channel support.
Since many synthetic, non-woven filtering materials have the property of memory, difficulties arise in retaining the marginal portion of the sheet of filtering material folded over the first frame and doubled back upon the central portion during the assembly of the filter unit. Specifically, pressure must be continually exerted against the marginal portion to retain it in its folded condition on the first frame, during reception of the second frame within the first frame. If the pressure were exerted manually, one or more hands would be required to exert the pressure and another hand would be required to insert the second frame into the first frame. Such manual assembly would increase the unit cost of each filter unit to an unacceptable level. In order to overcome this problem, heat, pressure and steam may be used to preform the marginal portion of the filtering material in its folded condition over the first frame. Additionally, or alternatively, adhesive may be used, preferably in the form of a double-sided adhesive strip connected to the sheet of filtering material adjacent the marginal portion. The strip is preferably provided with release paper that is removed to attach the filtering material to the first frame.
Once assembled, the marginal portion of the sheet of fitting material peripherally seals the filter unit by surrounding the first and second frames. The advantage of such 1 peripheral seal is that the filter unit may be mounted without additional brackets, gaskets, etc. For instance, the filter unit may be mounted within a close fitting duct with the filter unit being held in position solely by stops underlying the filter unit. The peripheral seal of the marginal portion prevents air leakage between the walls of the duct and the filter unit. Air leaks are also prevented between the first and second frames of the filter unit because the marginal portion is tightly held between the first and second frames.
Problems may arise in connection with the filter unit disclosed in my aforenoted prior European application. One problem is that when the filter unit is sufficiently elongated, the lengthwise portions of the frames may flex and bow. In an attempt to overcome such a possible drawback, cross-bracing was incorporated into the design. Such cross-bracing preferably comprises a pair of cross-braces attached to the second frame. The cross-braces have upturned end portions to form a set of four ear members. The ear members contribute to the alignment of the first frame and the sheet of filtering material with the second frame.
Another problem with the prior art frame assembly relates to the holding effect of the various components of the filter unit. Specifically, the marginal portion of the sheet of filtering material, over a period of time, tends to slip between the frames in response to air pressure acting on the sheet of filtering material. The end result of such slippage is that an outward bulge is produced in the filtering material in a direction of the air flow.